1. Technical Field
The present invention relates to a method of manufacturing a hollow stabilizer used for vehicles such as automobiles or the like, and more particularly, relates to a technology for increasing durability as much as possible.
2. Related Art
In recent years, stabilizers are mounted on most vehicles for the purpose of increasing roll rigidity. A stabilizer includes a solid stabilizer using a round bar as a material, and a hollow stabilizer using a pipe as a material for the purpose of saving weight. In the stabilizer, a spring constant is set for each type of automobile in order to obtain the roll rigidity required for the automobile. Accordingly, a wire diameter is selected for the solid stabilizer. In the case of changing the design from a solid stabilizer to a hollow stabilizer in order to reduce the weight of the automobile, the outer diameter of the hollow stabilizer necessarily becomes larger than the wire diameter of the solid stabilizer in order to maintain the roll rigidity. As a result, in the hollow stabilizer, in the case in which the same load as a load applied to the solid stabilizer is applied, the generated stress is larger in the hollow stabilizer. An increasing rate of stress depends on the thickness ratio (thickness t/outer diameter D) of the steel pipe constituting the pipe and a weight saving rate required in the hollow stabilizer.
Accordingly, in order to make it possible to change a design from a solid stabilizer to a hollow stabilizer, it is necessary to select an optimum steel pipe for the material, apply a heat treatment, contrive a bending method so as to minimize damage such as compression of a bent portion or the like, and to apply a shot peening to an outer surface so as to improve durability, and the like.
FIG. 6 shows a flow of a general method of manufacturing a conventional hollow stabilizer. An inexpensive electroseamed pipe is employed as a material of the hollow stabilizer, and the electroseamed pipe is cut at a predetermined length and is bent in a stabilizer shape. An NC bender or the like is used for the bending formation. In the NC bender, in order to restrict the compression of the material at the time of bending, the bending operation is executed while clamping an outer periphery of the electroseamed pipe by a bending roll and a bending piece moving along an outer periphery of the bending roll. Next, a heat treatment comprising a heating, a hardening, and a tempering is applied to a bent product, a shot peening is applied thereto, and thereafter, a coating process is applied thereto, whereby the hollow stabilizer is formed.
Most of the hollow stabilizer is formed by a material constituted by a thin electroseamed pipe having a thickness ratio (t/D) between 10 and 17%. Further more, a bead formed at the time of welding is left on an inner peripheral surface of the electroseamed pipe, constituting a defect. Accordingly, in order to make it possible to change the design to the hollow stabilizer formed by the electroseamed pipe as the material, it is necessary to consider an application of a heat treatment, a minimization of damage such as the compression of the bent portion due to the contrivance of the bending method, a detection and control of a bead position, an application of the shot peening and the like. Further more, in view of the material, it is necessary to employ a thick electroseamed pipe so as to make the outer diameter as close as possible to the outer diameter of the solid stabilizer, thereby reducing stress. Further more, in order to put the contrivance in the manufacturing step and the material side into practice as the design change of the hollow stabilizer, it is important how much the durability of the presently employed solid stabilizer satisfies the durability required by a user with a surplus.
Further more, as a result of improving the fatigue life of an outer surface portion in accordance with the shot peening, an inner surface portion of the hollow stabilizer to which the shot peening is not generally applied sometimes becomes a portion having the lowest fatigue strength in spite of generated stress being less than that at the outer surface. Since the thickness ratio of the conventional hollow stabilizer is generally about 15%, a difference in stress between the outer surface portion and the inner surface portion is small. As a result, a starting point of a fatigue failure frequently moves from the outer surface portion to the inner surface portion due to the shot peening. Accordingly, in order not to halve an effect obtained by applying the shot peening to the outer surface portion, it is desirable to make the design so that the inner surface portion is not the starting point of the fatigue failure.
Further more, the hollow stabilizer is generally bent in a cold state by the NC bender having a general purpose. In this process, the compression of the material in the bent portion causes a reduction in the fatigue strength. In addition, in order to improve a production tact, there is a requirement of bending respective portions to be processed in the heated electroseamed pipe in a lump by using a general bending mold. The process mentioned above can be mostly executed under the present circumstances because the material is significantly compressed.
In this case, as a method of manufacturing the hollow stabilizer employing the thick electroseamed pipe, there has been known a technique of drawing an electroseamed pipe having a small thickness and a large diameter in a cold state so as to set a thickness ratio between 18 and 20%, and applying a heat treatment and a shot peening after forming. However, in this technique, there is a problem in that the manufacturing cost becomes rather high because the drawing process is executed.
Further more, in Japanese Unexamined Patent Publication No. 2000-233625, there is described a manufacturing method of compressing an electroseamed pipe having a large diameter to a thickness ratio less than 0.2 in a hot state so as to form a raw pipe, drawing the raw pipe in a cold state so as to make the thickness ratio between 0.2 and 0.27, and applying a strain removing annealing and a shot peening after forming. However, in accordance with the manufacturing method, the manufacturing cost becomes rather high because the drawing process is executed, and there is a problem in that the maximum thickness is limited due to a limit in a cold drawing capacity and a range of selection in the design is narrow.